A couple of stories at BBC and Information Week about Windows Phone 7 SENDING (Hello !?!?) over a gigabyte of data per month simply sitting in your holster. This is where you get horror stories about people who take a phone with them to the US or Europe, never make a phone call, and get a huge multi-thousand dollar bill when they get back home.
"On December 22nd I received an email from AT&T saying that I was close to my 2GB data limit which truly shocked me as I feel I do not use data that much. I went and looked at my AT&T account online and noticed that my phone was sending huge chunks of data seemingly in patterns. For instance on November 21-24 it sent between 30 and 50 MB of data at 10:41pm each day and Dec 1-4 it sent between 30 and 50 MB of data at 9:41am each day. On December 23rd I turned on airplane mode so my phone could no longer send data. I turned airplane mode off briefly on December 23rd and the phone sent 400 MB of data. I called AT&T yesterday, December 28th, but they said that there was nothing that they can do to figure out what was happening on my phone."